Present amplifiers for long wavelength PIN-GaAs FET receivers use high-impedance front-ends. The front-end bias resistor is made as large as possible to maximize the signal and to minimize the thermal noise. Unfortunately, this reduces the frequency bandwidth and the signal is integrated by the input capacitance. Long strings of 1's and 0's cause the amplifier to saturate at relatively low input levels thereby restricting the dynamic range.
The most obvious scheme for overcoming this dynamic range problem is to attach a current shunt (field effect transistor, bipolar transistor or a diode) at the input node of the amplifier. The scheme is shown in FIG. 1. As the input signal from the PIN photodiode is increased, the shunt in turned on and the current into the amplifier is limited to a level below saturation. Unfortunately, any connection to the input node adds capacitance to the front-end and a sensitivity penalty is incurred.